Conventionally, simultaneous decoration and injection molding has often been employed for decoration of a resin molding having a surface of complicated shape (e.g., a three-dimensional curved surface) (see, for example, Patent Documents 1 and 2). Simultaneous decoration and injection molding is a method in which, during the course of injection molding, a decorative sheet placed in a mold is united with a molten resin injected into the cavity of the mold, whereby the surface of a resin molding is decorated. In general, the simultaneous decoration and injection molding method is roughly classified into a laminate decoration method and a transfer decoration method, according to the structure of a decorative sheet to be united with a resin molding.
In the laminate decoration method, the whole decorative sheet including a base film and a decorative layer provided thereon is laminated on the surface of a resin molding so that the decorative sheet is united with the molding. The decorative sheet to be employed is a decorative laminate sheet.
An example laminate-type simultaneous decoration and injection molding method will now be described with reference to FIGS. 2 and 3. FIGS. 2 and 3 are explanatory views showing an example of the molding step of the simultaneous decoration and injection molding method. A simultaneous decoration and injection molding apparatus 60 includes a female mold 70, and a male mold 80 which is disposed laterally to the mold 70 so as to face the mold 70. The female mold 70 has a cavity 72 having a shape corresponding to the contour of a molding to be formed, and includes suction holes 74 which extend throughout the mold 70 and are open to the cavity 72. The female mold 70 is adapted to be reciprocally moved by means of a reciprocating apparatus 75 formed of, for example, a fluid-pressure cylinder in directions toward and away from the male mold 80. The male mold 80 has a core section 82 which is inserted into the cavity 72, the core section 82 including a gate 84 therein for injecting a molten resin in the cavity. If necessary, a reciprocally movable heating plate 90 is provided between the female mold 70 and the male mold 80.
In order to perform decoration simultaneously with injection molding by means of the apparatus 60, firstly, a decorative sheet 100 is provided on the female mold 70 so as to face the male mold, and, if necessary, the decorative sheet 100 is heated and softened at an appropriate temperature by means of the heating plate 90. Subsequently, the decorative sheet 100 is sandwiched by the female mold 70 and the heating plate 90, to thereby close the opening of the cavity 72. The thus-closed cavity is evacuated through the suction holes 74 provided in the female mold 70, and, if desired, pressurized air is supplied to the sheet through a vent provided in the heating plate 90. Both molds are generally heated to about 30 to about 50° C.
Through this procedure, as shown in FIG. 3, the decorative sheet 100 is stretched along the inner wall of the cavity 72 so as to come into close contact therewith. This procedure is generally called “preforming,” in which a softened sheet is generally stretched up to about 200%. Subsequently, the heating plate 90 is withdrawn, and, as shown in FIG. 3, the female mold 70 is moved toward the male mold 80 so that the molds are united together, followed by mold clamping. Thereafter, a fluidized resin molding material P is injected, through the gate 84 provided in the male mold 80, into a cavity formed between the female mold 70 and the male mold 80 until the cavity is filled with the material, and thus injection molding is performed.
Through this procedure, the decorative sheet 100 placed in the female mold 70 is attached to and united with the thus-injected resin. After completion of injection molding, the molds are separated from each other, to thereby remove, from the molds, a molding whose outer surface is coated with the decorative sheet 100. Thus, laminate-type decoration is completed.
In the aforementioned simultaneous decoration and injection molding method, important requirements for formation of a good molded product are that the decorative sheet 100 can be stretched along the inner wall of the cavity 72 so as to come into close contact therewith during the course of preforming or molten resin injection (i.e., moldability of the sheet), and that the decorative sheet is not deformed through stretching to an extent exceeding that necessary for fitting the sheet to the shape of the mold, which deformation may be caused by, for example, vacuum/pressure effect, or tension due to the pressure or shear stress of the molten resin. These requirements are particularly important in the case of molding by means of a mold having a large depth, since the decorative sheet is subjected to deep drawing.
In the case of deep-draw molding (i.e., in the case where the percent stretching of a decorative sheet is high), generally, such a resin-injection-molded product is required to be decorated so as to attain a drawn shape corresponding to a percent stretching of about 200 to about 400% of the decorative sheet. In order to satisfy such a requirement, there have been proposed, for example, a decorative sheet formed of a material which has a yield point and which receives stress of a certain level or more after the yield point (see claims of Patent Document 3), and as well a decorative sheet for molding formed of two or more laminated films, the sheet exhibiting a peeling strength of a certain level or more at the interface between the sheet and a resin to be molded, wherein a transparent acrylic film is provided on the film which is bonded to the resin, and a pattern is formed between the acrylic film and the film which is bonded to the resin (see claims of Patent Document 4). In order to sufficiently conform to the irregular surface shape of the molded product, the aforementioned decorative sheet for molding exhibits an elongation at break (percent stretching) of about 150 to 200% or more, or about 400% as maximum in the temperature range of 100 to 120° C., to which the decorative sheet is heated during the preforming or injection molding.
In general, a laminate-type decorative sheet having an area exceeding the surface area of a molded product is bonded to the molded product. Therefore, the decorative sheet must be subjected to a step called “trimming” for cutting or removing an excess portion provided along the periphery of the molded product. Such trimming is performed at about 0 to about 40° C. (generally at room temperature (about 25° C.)). The aforementioned decorative sheet disclosed in Patent Document 3 or 4, which exhibits an elongation at break (percent stretching) of about 150 to about 200% or more as measured at 100 to 120° C., generally exhibits an elongation at break (percent stretching) as high as 20% or more as measured at room temperature. When such a decorative sheet exhibiting an elongation at break (percent stretching) as high as 20% or more as measured at room temperature is employed, the decorative sheet poses a problem in that an excess decorative sheet portion provided along the periphery of a molded product fails be cut successfully during the course of trimming, and thus the excess decorative sheet portion remains on the molded product, or the decorative sheet is exfoliated at an end portion of the molded product.
Meanwhile, when a decorative sheet exhibiting low elongation at break as measured at 100 to 120° C. is employed, trimming of the sheet can be readily performed, since the sheet generally exhibits low elongation at break as measured at room temperature. However, when such a decorative sheet is employed for deep-draw molding, the sheet fails to sufficiently conform to surface irregularities of a molded product, and in some cases, the sheet is broken during the course of printing.
Patent Document 1: Japanese Patent Publication (kokoku) No. S50-19132
Patent Document 2: Japanese Patent Publication (kokoku) No. S61-17255
Patent Document 3: Japanese Patent No. 2690258
Patent Document 4: Japanese Patent No. 2965973